The Pythagorean Theorem is a fundamental concept in geometry that states that in a right-angled triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the other two sides. It can be used to find the length of the hypotenuse or one of the other two sides of a right-angled triangle. The Pythagorean Theorem is used in many real-world applications, such as construction, engineering, and navigation.